Several studies have proven that BL Lacs exhibit high amplitude and rapid
variability on timescales that can vary from hours to months, and so the nature of
the X-ray spectra in BL Lacs is variable and shows a complex behaviour
[1]}, [2]}, [3]}. The
shape of the X-ray spectrum can provide information that can be used to reveal the emission components, as X-ray emission probably originates in the inner
parts of the relativistic jet. The transition in the SED between synchrotron
and IC in BL Lacs is located with the X-rays, and therefore characterising the
spectra in this band where both processes can contribute to the X-rays is of
special relevance. When the mechanism responsible for the emission is
synchrotron, we observe a steep power-law energy distribution (\(\alpha > 1\) ,
\(F \propto \nu ^{-\alpha }\) ) due to the tail of its spectrum, while if IC is the
dominant process we observe a flat component (\(\alpha < 1\) ). In the SED of
HSPs, the X-ray radiation comes from the high-energy end of the synchrotron,
whereas in LSPs it comes from Compton scattering.
